Tungsten carbide compositions have been useful for providing hard, durable surfaces. One particularly important application for such surfaces is tools, such as calender rolls for use in manufacturing. However, many tools require surfaces with very smooth finishes. In particular, calender rolls used to apply magnetic coatings on tape have very exacting smoothness requirements, for example, an RMS (root-mean-square) surface roughness of about 25 nm (about 1 microinch) or less. The RMS surface roughness is a measurement of the typical surface structure found on the object. Surface deformities in the calender roll, such as pits and peaks (e.g., particles sticking out of the surface) will be transferred to the magnetic coating during the coating process. However, the surface of the magnetic tape, after the magnetic coating has been applied, should be very smooth to reduce noise caused by the surface deformities.
Other manufacturing processes also require calender rolls that, although they often do not need the precision of magnetic tape rolls, should have a relatively smooth surface. Furthermore, smooth surfaces made using a hard, durable material may be useful in other applications, such as optics.
Currently, calender rolls in the magnetic film industry have chrome coatings, which also have relatively smooth surfaces. However, these coatings are relatively soft and wear out rapidly. Furthermore, chrome surfaces also tend to have cracks that give the surface the appearance, under a microscope, of a dry lake bed. These cracks may be transferred to the magnetic tape. Moreover, chrome has been recognized to be environmentally hazardous and is now regulated. Thus, there is a need for an alternative.
One potential substitute for chrome is tungsten carbide, which forms a harder and more durable surface coating for calender rolls and other tools. Suitable tungsten carbide coatings often include up to 40 wt. % of other metals, such as chromium, cobalt, and nickel. The addition of these metals often gives the tungsten carbide coating improved properties, such as durability, hardness, and enhanced bond strength with the substrate.
However, these tungsten carbide coatings are often sensitive to current finishing methods and there has been difficulty achieving the necessary surface roughness requirements; in particular, when a relatively uniform profile must be obtained. One danger when trying to finish tungsten carbide is overworking (e.g., using many passes to finish the surface). Overworking a tungsten carbide surface typically increases the porosity of the surface as portions of the surface are removed. This results in an increased RMS surface roughness and possibly introduction of pits into the surface. A method is needed for finishing tungsten carbide surfaces to obtain the desired smoothness without overworking the surface and defeating the desired goal.